1. Field of the Invention
The present invention relates to an image recording material, provided on a support and comprising a photosensitive/heat sensitive layer with improved solubility to an alkaline developer by exposure. More particularly it relates to a so-called direct plate-making positive image recording material usable for direct plate-making by scanning a laser beam with a high energy density, such as an infrared laser beam, based on digital signals from a computer or the like.
2. Description of the Related Art
Lasers have advanced remarkably recently, and especially compact and high powered solid lasers and semiconductor laser, having light emissions in the near infrared to infrared regions, have become readily available. Recording by changing the solubility of photosensitive resins by exposure using such high power lasers (generally, the exposure energy density is greater than 5 kW to 10 kW/cm2) is called heat mode recording or thermal recording. This way of recording has recently been drawing attention in the field of planographic printing for recording in direct plate-making methods for directly producing master plates from digital data of a computer or the like.
Specifically, commercialized and made available are: thermal positive type planographic printing plate precursors, comprising photosensitive layers provided on a support and whose solubility is increased by high power laser exposure (called positive type photosensitive layers); or thermal negative type planographic printing plates comprising photosensitive layers provided on a support whose solubility is decreased by high power laser exposure (called negative type photosensitive layers).
Positive type image recording materials for infrared lasers for direct plate making are image recording materials obtained by adding to an alkali-soluble resin a substance which absorbs light and generates heat, and a positive type photosensitive compound, such as a quinonediazido compound. The positive type photosensitive compound works as a dissolution inhibitor by effectively decreasing the solubility of the aqueous alkaline solution-soluble resin in the image areas. And in the non-image areas, the photosensitive compound is decomposed by heat and-therefore does not show dissolution inhibition, and the aqueous alkaline solution-soluble resin becomes removable by development to form images. A variety of alkali-soluble resins and dissolution inhibitors are proposed (reference to Japanese Patent Application Laid-Open Nos. 7-285275, 10-268512, and Japanese Patent Application National Publication No. 11-506550).
However, known photosensitive compositions used as recording layers of known thermal positive type planographic printing plate precursors have insufficient difference of the solubility to developers between exposed portions and non-exposed portions (solubility discrimination) and excess development or defective development attributed to variation of conditions of use is easily caused and accordingly, problems often results where the contrast of images developed after exposure becomes insufficient.
As a method for improving the image discrimination of a positive type planographic printing plate material, techniques of adding phenolic hydroxyl-containing compounds have been proposed (reference to Japanese Patent Application Laid-Open (JP-A) No. 2000-241966). However, whilst the phenolic hydroxyl-containing compounds improve the removability (solubility) of the non-image portions to alkaline developers, on the other hand, the compounds also, at the same time, increase the solubility of the image portions. This means that the sharpness of images deteriorates and this tendency is particularly noticeable in the case of thin lines and dotted image regions with low density coverage. In such situations, an improvement in image recording materials in terms of contrast has been desired.